Programming in C in a UNIX environment

Transcription

1 Programming in C in a UNIX environment Software Development Lifecycle Understand the requirements Develop the algorithm/approach Write the code Debug the code Test the code Iterate until you get it right 2 1

2 Programming Tools Several Integrated Development Environments (IDEs) Visual Studio, Netbeans, Eclipse, jgrasp IDEs are great but there is a learning curve But you don t need to use an IDE. You need a - Text Editor -- use vi or emacs or jedit (not PICO!!) - Compiler -- use gcc in conjunction with the Make utility - Debugger -- use gdb/insight Become familiar and comfortable with UNIX See useful links on class web page Links to several tutorials on UNIX, gdb, make, emacs, vi, etc. 3 Program with multiple files #include <stdio.h> #include mypgm.h void main(void) myproc(); hw.c #include <stdio.h> #include mypgm.h void myproc(void) mydata=2;... /* some code */ mypgm.c Library headers Standard User-defined void myproc(void); int mydata; mypgm.h 4 2

3 Externs #include <stdio.h> extern char user2line [20]; /* global variable defined in another file */ char user1line[30]; /* global for this file */ void dummy(void); void main(void) char user1line[20]; /* different from earlier user1line[30] */... /* restricted to this func */ void dummy() extern char user1line[]; /* the global user1line[30] */... 5 Example to show separate compilation and make Consider a program which is split into several files with each function in a separate file (Note - this is just for illustrating the use of make!!) Defs.h Main.c ReadLine.c WordCount.c ExternVars.h PrintLine.c UpdateCounts.c 6 3

4 Example cont d ExternVars.h extern char Line[MaxLine]; extern int NChars,NWords,NLines,LineLength; Defs.h #define MaxLine Main.c /* introductory C program implements (a subset of) the Unix wc command -- reports character, word and line counts; in this version, the "file" is read from the standard input */ #include "Defs.h extern int ReadLine(),WordCount(); char Line[MaxLine]; /* one line from the file */ int NChars = 0, /* number of characters seen so far */ NWords = 0, /* number of words seen so far */ NLines = 0, /* number of lines seen so far */ LineLength; /* length of the current line */ main() while (1) LineLength = ReadLine(); if (LineLength == 0) break; UpdateCounts(); printf("%d %d %d\n",nchars,nwords,nlines); 8 4

5 PrintLine.c #include "Defs.h #include "ExternVars.h PrintLine() Code not shown Readline.c #include "Defs.h #include "ExternVars.h int ReadLine() Code not shown WordCount.c #include "Defs.h #include "ExternVars.h int WordCount() Code not shown UpdateCounts.c #include "Defs.h #include "ExternVars.h extern int WordCount(); UpdateCounts() Code not shown 9 Makefile Makefile 3 WC: Main.o ReadLine.o WordCount.o UpdateCounts.o PrintLine.o 4 cc -g -o WC Main.o ReadLine.o WordCount.o UpdateCounts.o \ PrintLine.o 5 6 Main.o: Main.c Defs.h ExternVars.h 7 cc -g -c Main.c 8 9 ReadLine.o: ReadLine.c Defs.h ExternVars.h 10 cc -g -c ReadLine.c WordCount.o: WordCount.c Defs.h ExternVars.h 13 cc -g -c WordCount.c UpdateCounts.o: UpdateCounts.c Defs.h ExternVars.h 16 cc -g -c UpdateCounts.c PrintLine.o: PrintLine.c Defs.h ExternVars.h 19 cc -g -c PrintLine.c Note: The makefile does not really have line numbers as shown 10 5

6 Chapter 15 Debugging Debugging with High Level Languages Same goals as low-level debugging Examine and set values in memory Execute portions of program Stop execution when (and where) desired Want debugging tools to operate on high-level language constructs Examine and set variables, not memory locations Trace and set breakpoints on statements and function calls, not instructions...but also want access to low-level tools when needed 12 6

7 Types of Errors Syntactic Errors Input code is not legal Caught by compiler (or other translation mechanism) Semantic Errors Legal code, but not what programmer intended Not caught by compiler, because syntax is correct Algorithmic Errors Problem with the logic of the program Program does what programmer intended, but it doesn't solve the right problem 13 Syntactic Errors Common errors: missing semicolon or brace mis-spelled type in declaration One mistake can cause an avalanche of errors because compiler can't recover and gets confused main () missing semicolon int i int j; for (i = 0; i <= 10; i++) j = i * 7; printf("%d x 7 = %d\n", i, j); 14 7

8 Semantic Errors Common Errors Missing braces to group statements together Confusing assignment with equality Wrong assumptions about operator precedence, associativity Wrong limits on for-loop counter Uninitialized variables missing braces, main () so printf not part of if int i int j; for (i = 0; i <= 10; i++) j = i * 7; printf("%d x 7 = %d\n", i, j); 15 Algorithmic Errors Design is wrong, so program does not solve the correct problem Difficult to find Program does what we intended Problem might not show up until many runs of program Maybe difficult to fix Have to redesign, may have large impact on program code Classic example: Y2K bug only allow 2 digits for year, assuming

9 Debugging Techniques Ad-Hoc Insert printf statements to track control flow and values Code explicitly checks for values out of expected range, etc. Advantage: No special debugging tools needed Disadvantages: Requires intimate knowledge of code and expected values Frequent re-compile and execute cycles Inserted code can be buggy Source-Level Debugger Examine and set variable values Tracing, breakpoints, single-stepping on source-code statements 17 Source-Level Debugger main window of Cygwin version of gdb 18 9

10 Source-Level Debugging Techniques Breakpoints Stop when a particular statement is reached Stop at entry or exit of a function Conditional breakpoints: Stop if a variable is equal to a specific value, etc. Watchpoints: Stop when a variable is set to a specific value Single-Stepping Execute one statement at a time Step "into" or step "over" function calls Step into: next statement is first inside function call Step over: execute function without stopping Step out: finish executing current function and stop on exit 19 Source-Level Debugging Techniques Displaying Values Show value consistent with declared type of variable Dereference pointers (variables that hold addresses) See Chapter 17 Inspect parts of a data structure See Chapters 19 and

11 Assignment 1 - string input and manipulation The handout for Assignment 1 says that you should use fgets() to read lines input by the user fgets() does not report a problem if the input string includes a NULL character Usually not a problem, but a better alternative is getline() - supported by GNU C standard I/O library You are also required to use several string manipulation functions See the man pages for each function to see more details about how a function works. For example, type man fgets 21 Example of fgets() #include <stdio.h> int main() int input_character; FILE *my_stream; char my_filename[] = "snazzyjazz.txt"; char my_string[100]; my_stream = fopen (my_filename, "w"); fprintf (my_stream, "Hidee ho!\n"); /* Close stream; skip error-checking for brevity of example */ fclose (my_stream); my_stream = fopen (my_filename, "r"); fgets (my_string, 100, my_stream); /* Close stream; skip error-checking for brevity of example */ fclose (my_stream); printf ("%s", my_string); return 0; 22 11

12 Example: using getline() #include <stdio.h> int main() int bytes_read; int nbytes = 100; char *my_string; puts ("Please enter a line of text."); /* These 2 lines are the heart of the program. */ my_string = (char *) malloc (nbytes + 1); bytes_read = getline (&my_string, &nbytes, stdin); if (bytes_read == -1) puts ("ERROR!"); else puts ("You typed:"); puts (my_string); return 0; 23 12